The latest standards from the International Telecommunication Union (ITU) for the Flexible Optical Transport Network (FlexO) provide for data transmission at rates from 400 to 800 gigabits per second (Gbit/s).

New features also include regeneration to extend FlexO’s reach and a new class of Ethernet-optimized FlexO interfaces.

ITU first defined FlexO in 2017 to meet industry’s need for short-reach optical transport network (OTN) interfaces with data rates beyond 100 Gbit/s. The organization’s FlexO standards now underpin all OTN interfaces capable of supporting rates over 100 Gbit/s, whether short-reach client interfaces or long-reach coherent line interfaces.

ITU’s first FlexO standard, G.709.1, provided a frame format optimized for short-reach point-to-point applications using inverse multiplexing to carry beyond-100G signals over multiple 100G modules.

This scalable design, based on multiple 100G instances, enables data rates from 100 Gbit/s up to multiple terabits per second.

Updates to ITU’s FlexO standards have added support for modules using 200G, 400G, and 800G bit rates, as well as multi-span metropolitan-area applications with regenerators.

ITU’s FlexO standards provide the common elements leveraged by the coherent optical interface specifications developed by OIF (Optical Internetworking Forum), OpenZR+ multi-source agreement (MSA), and OpenROADM MSA.

New FlexO features

New frame formats and interfaces, optimized for the support of Ethernet at rates up to 800 Gbit/s, provide for longer reach interfaces capable of data transmission at 400 Gbit/s with DPSK (differential phase-shift keying) modulation and 800 Gbit/s with 16-QAM (quadrature amplitude modulation).

FlexO regeneration, enabled by new procedures for overhead and operations, administration, and management, extends FlexO’s reach by introducing the capability to regenerate a FlexO interface or signal without terminating the FlexO layer.

Ethernet-optimized FlexO interfaces, describing overhead and mapping procedures specific to the mapping of Ethernet in FlexO signals, support applications such as Internet Protocol over Wavelength Division Multiplexing (WDM) networks and point-to-point or point-to-multipoint Ethernet.

Restructured FlexO standards suite

Foundational FlexO building blocks are now all outlined by the ITU standard G.709.1.

The new structure of ITU’s FlexO standards will make it simpler for other standards bodies or multi-source agreements (MSAs) to reference the parts of the base FlexO specifications relevant to their target application.

This is particularly important to the continued evolution of FlexO.

ITU’s FlexO standards provide for the short-reach client interfaces of Ethernet or the long-reach coherent line interfaces over WDM. Whether multi-vendor interoperable or single-vendor proprietary, FlexO interfaces all take common elements from ITU standards.

A common set of base specifications, referenced by other standards bodies or MSAs for specific applications, will encourage the development of chips and modules that can be reused across a wide range of applications.

The restructured FlexO standards suite includes:

• G.709.1 Flexible OTN common elements
• G.709.3 Flexible OTN B100G long reach interfaces
• G.709.5 Flexible OTN short reach interfaces
• G.709.6 Flexible OTN B400G long reach interfaces

The new standards suite is set to be published in early 2024.

FlexO standards are developed by ITU-T Study Group 15 (Transport, access and home). Learn more about the origins, evolution, and latest features of FlexO from Q11/15 Rapporteur Steve Gorshe and FlexO Editor Sebastien Gareau: Play the ITU video interview.